1. Field of the Invention
This invention relates to a heterojunction bipolar transistor.
2. Related Art
There is continuing and increasing interest in improving the operating speed of bipolar transistors in microcircuits, particularly in the context of VLSI circuits for high-speed and ultra high-speed logic applications.
One route to such speed improvements, which has been relentlessly pursued to the practical limits of existing technology, is the reduction in feature size of the devices. In connection with this, new self-aligned processes, which remove the need to provide for alignment tolerances, continue to be developed.
An alternative route to achieving higher operating speeds is to use gallium arsenide (GaAs) or other related III-V compound semiconductors in place of silicon, since the electron mobility and saturation velocity are higher in GaAs and the other III-Vs than in silicon. Unfortunately, although at first sight it appears possible to achieve a respectable speed increase by producing GaAs versions of bipolar transistors which are conventionally made of silicon, such an approach does not in fact give rise to any significant improvement in performance. The problem is that, in such devices, it is necessary to keep the base width small, and use only low levels of base doping, with the result that the base resistance tends to be high. In order to keep the base resistance acceptably low, high hole mobility is needed. Unfortunately, although electron mobility in GaAs is high (which is useful for giving short emitter-collector transit times), hole mobility is less than half that of silicon, resulting in excessive base resistance.
An approach to making high speed bipolar devices in GaAs which gets round the limitation of poor hole mobility is to produce the device as a heterojunction bipolar transistor (HBT) using aluminium gallium arsenide (AlGaAs) and gallium arsenide (GaAs).
In an HBT, the emitter is formed of a material having a larger energy gap than the base, whereby injection of holes from the base into the emitter is prevented and it becomes possible to use heavy base doping, and hence very thin (1,000 .ANG.) base widths, without excessive base resistance.
Unfortunately, although HBTs theoretically offer quite exceptional performance (for more details of which see the review paper by Herbert Kroemer in PROC.IEEE, Vol. 70, No. 1, Jan. 1982), they have proved exceedingly difficult to make.
The bulk of the work done on HBTs has involved the use of AlGaAs and GaAs, in part because of the superior electronic properties of GaAs, but also because it is possible to make very high quality AlGaAs-GaAs heterojunctions. Unfortunately, fabrication of GaAs HBTs is very complex because of the difficulties inherent in GaAs processing. In particular, the absence of a native oxide means that processing is limited to etching and non-selective deposition, which in turn means that GaAs devices are essentially non-planar. It can be seen, therefore, that there are appreciable costs associated with the speed advantages offered by GaAs. It is to the reduction of these costs, while realising the benefits of improved speed, that work on HBTs has been directed in recent years.
The comparative ease with which silicon can be processed and the ubiquitous nature of silicon processing together provide a strong incentive for the development of an HBT which is compatible with existing silicon technology. A few silicon based wide-gap emitter transistors have been reported. For example, gallium phosphide-silicon has been tried with disappointing results; and, with a heavily-doped "semi-insulating polycrystalline" silicon emitter on a silicon base (the polycrystalline/amorphous silicon having a wider band gap than that of the single crystal silicon of the base), the results achieved have been better, but are still a long way short of those obtained with AlGaAs/GaAs. Consequently, research on HBTs continues to be centered on the use of GaAs.